Schizophrenia is a severe psychiatric disorder affecting approximately 1% of the world's population. Clinical symptoms of schizophrenia include delusions, auditory hallucinations, disorganized thoughts and speech, social withdrawal, lack of motivation, and cognitive dysfunction such as disorganized thinking and memory impairments. This disorder is believed to be caused by a combination of neurological defects including dopamine and serotonin levels, and inhibitory interneuron deficiencies (Freedman, 2003, New Eng. J. Med., 349(18): 1738-1749). Schizophrenia can be treated with drugs which target neurotransmitters and receptors, commonly referred to as anti-psychotic or neuroleptic drugs.
One class of anti-psychotic drug termed “atypical anti-psychotics” or “second generation anti-psychotics” includes the benzisoxazole derivative, risperidone (I). Risperidone, marketed under Risperdal® by Janssen-Cilag in the United States, targets the serotonin (5-HT2A) and dopamine (D2) receptors, blocking the uptake of their respective neurotransmitters (Package-Insert-Risperdal, 2009, Janssen Cilag). Risperidone is metabolized in humans by cytochrome P450 isoform CYP2D6 to the biologically active 9-hydroxy metabolite, paliperidone (II). Since both have been shown to have similar in vitro efficacy, together they constitute an “active moiety,” and should be monitored collectively (Mannens et al., 1993, Drug Met. & Disp., 21(6): 1134-1141). Paliperidone itself has been approved by the FDA recently as a treatment for schizophrenia, marketed under Invega® by Janssen Pharmaceutica (Package-Insert-Invega, 2009, Janssen Pharmaceutica).
Risperidone has the following formula:

Paliperidone has the following formula:

The “active moiety” of risperidone and paliperidone has been shown to have up to a 13-fold inter-patient variability in plasma steady-state concentrations and that this variability can impact efficacy and safety (Spina et al., 2001, Psychopharmacol., 153(2): 238-243; Aravagiri et al., 2003, Ther. Drug Monitor., 25(6): 657-664; Riedel et al., 2005, Eur. Arch. Psych. and Clin. Neurosci., 255(4): 261-268).
Since efficacy of risperidone and paliperidone is improved at higher trough levels and that the drug exhibits wide intra-patient pharmacokinetic variability monitoring concentrations of this drug in blood and adjusting to target levels would be of value in increasing efficacy and minimizing toxicity (Raggi et al., 2004, Med. Chem. Rev., 1: 299-316; Musenga et al., 2009, Curr. Med. Chem., 16(12): 1463-1481). The degree of intra- and inter-individual pharmacokinetic variability of risperidone and its derivative has been reported to be 13-fold and is impacted by many factors, including:                Age        Weight        Organ function        Drug-drug interaction        Genetic regulation        Compliance        
As a result of this variability, equal doses of the same drug in different individuals can result in dramatically different clinical outcomes. The effectiveness of the same dosage of risperidone and paliperidone varies significantly based upon individual drug clearance and the ultimate serum drug concentration in the patient. Therapeutic drug management would provide the clinician with insight on patient variation in drug administration. With therapeutic drug management, drug dosages could be individualized to the patient, and the chances of effectively treating the disorder without the unwanted side effects would be much higher.
In addition, therapeutic drug management of risperidone and paliperidone would serve as an excellent tool to ensure compliance (Valenstein et al., 2006, J. Clin. Psych., 67(10): 1542-1550; Treur et al., 2009, BMC Health Serv. Res., 9: 9) in administering anti-psychotic drugs with the actual prescribed dosage and achievement of the effective serum concentration levels. Routine therapeutic drug management of risperidone and paliperidone would require the availability of simple automated tests adaptable to general laboratory equipment. The use of liquid chromatography (LC) with UV or mass spectroscometry detection to determine the concentration of risperidone and paliperidone in human blood and plasma has been described (Balant-Gorgia, et al., 1999, Ther. Drug Monitor., 21(1): 105-115; Schatz et al., 2000, Pharmacol., 60(1): 51-56; Frahnert et al., 2003, J. Chrom. B, 794(1): 35-47; Zhang et al., 2008, Biomed. Chrom., 22(7): 671-687). These methods are labor intensive, requiring liquid-liquid or solid phase extractions, use expensive equipment and are not amenable to routine clinical laboratory use. To date, there are no immunoassays for measuring risperidone and/or paliperidone in human biological fluids of patients treated with these anti-psychotic agents.
As seen from the foregoing, there are no immunoassays for determining the presence and/or quantifying the amount of risperidone and paliperidone in human biological fluids. Routine therapeutic drug management of risperidone and paliperidone by immunoassays would provide simple automated tests adapted to standard laboratory equipment. However, in order to provide such immunoassays, antibodies selective to risperidone and paliperidone must be produced. The derivatives and immunogen used in this assay must impart through these corresponding antibodies produced selective reactivity to risperidone and paliperidone without any substantial cross reactivity to other therapeutically active or inactive, or pharmacologically active or inactive metabolites of risperidone and paliperidone. In order to be effective in monitoring drug levels, the antibodies should be selective to risperidone and paliperidone and not cross reactive with pharmaceutically or pharmacologically inactive metabolites of risperidone and paliperidone. The principle pharmaceutically or pharmacologically inactive metabolites are 7-hydroxy-risperidone (V) or N-dealkyl-risperidone (VI) (Mannens, et al., 1993, Drug Met. & Disp., 21(6): 1134-1141; He et al., 1995, Int. Clin. Psychopharmacol., 10(1): 19-30), which metabolites have the formulae:

In order to be effective in an immunoassay, these antibodies should have substantially low reactivity to the pharmaceutically or pharmacologically inactive metabolite, 7-hydroxy-risperidone V, while having substantially no or any reactivity to the other pharmaceutically or pharmacologically inactive metabolites, particularly N-dealkyl-risperidone VI. This is so since the metabolite, 7-hydroxy-risperidone V is only produced from risperidone and paliperidone in very small amounts.